The present invention relates to a novel glass composition which exclusively comprises oxides which at low temperatures form volatile fluorides and its use in micro structuring methods.
Today, within the scope of the permanently increasing miniaturisation, the development of micro optics is an intensively examined field of development. A potential technology for the structuring of intermediate products is reactive ion etching (RIE). In this case, in bodies of silicon dioxide, such as for example wafers, the desired micro structures are generated in a plasma discharge reactor through a masking technique.
In the case of micro structuring of SiO2, volatile silicon fluoride which does not condense on the article due to the state of aggregation is formed with the use of for example CF4 or another fluorine-containing gas. SiO2 is a glass having many positive properties and may further be obtained in different geometries. Thus, SiO2 is a suitable material for this method. But currently, SiO2 is the only material which is available in bulk form for this micro structuring method. As a single component, it cannot fulfil the entirety of many demands. One significant disadvantage is e.g. that SiO2 melts only at very high temperatures (Ts=1.723° C.) and thus, the production generates high costs. Certain geometries can only be realized by costly methods of cold reprocessing. Furthermore, the low coefficient of thermal expansion of SiO2 glass which is 0.5 ppm/K is disadvantageous, when it should be linked with other materials, e.g. semiconductors, for example in the so-called “wafer level assembly”, wherein wafers of glass and of semiconductor materials should be linked and then processed together.
Finally, the low refractive index of SiO2 glass is disadvantageous, when lenses with high numerical aperture should be produced.
In this case, glasses would be preferable which have expansion coefficients in the range of the values of semiconductor materials, namely ca. 2 to 5 ppm/K, which have suitable refractive indices and which further can be produced in different required geometries.
There are a series of commercially available glasses which fulfil the demands regarding geometry, expansion coefficient and refractive index, but they all have the disadvantage that by the use of reactive ion etching which is also called RIE they form solid fluorides which after their formation immediately condense on the body to be structured. This applies to all known bulk glasses—with exception of SiO2—, since most elements of the periodic table form solid fluorides at room temperature. For example borosilicate glasses, such as for example Duran®, are available as flat glasses and can be linked well with Si, because of their expansion coefficient of 3.3 ppm/K, but in this case there is a disadvantage which lies in the fact that with the use of the RIE method the generated micro structures have low quality, since the oxides of Al, Na and K form solid fluorides during the reaction with F. There are investigations to overcome this disadvantage by a combined etching by means of the RIE method and sputtering (E. Metwalli and C. P. Pantano, Reactive ion etching of glasses: Composition dependence, Nucl. lnstr. Meth. Phys. Res. B 207 (2003) 21-27) which however is costly and undesired and in addition, nearly no micro structures with optical quality can be produced with this. Furthermore, the achievable etching rates decrease to a value which is higher than the half of the value of SiO2, thus the economical desirability of the method is remarkably lower.
Analogously, another report in the literature (Journal of Non-Crystalline Solids 342, (2004), 110-115) shows that during the reactive ion etching very complex processes take place and have to be managed, when it should be tried to microstructure alkali- or alkaline earth-containing glasses or glass ceramics by means of RIE.
From the state of the art (see J. Electrochem. Soc., Vol. 138, No. 9, pages 2836-2838, September 1991), glass systems are known which contain SiO2, GeO2, B2O3 and P2O5. They could be used in the micro linking technique in the form of reflow processes, namely for the demand of glasses which have a reflow temperature which is remarkably lower than that of SiO2. However, these glasses contain at least 71% by mole of SiO2 and are featured by glass transition temperatures (Tg) of higher than 800° C. Although the processing temperatures of these glasses are not mentioned in the literature, it can certainly be expected that all the glasses have processing temperatures which are clearly higher than 1.400° C. and thus cannot be melted in big conventional melting aggregates.